The present invention relates to a graphics display controller for managing a display memory and drawing figures on a screen of a raster-scan display and, more particularly, to such a controller having a clipping function for clipping or limiting a drawing area of figures on a display screen.
A clipping function is used to allow figures to be drawn only into an inside (or outside) area of a rectangle which has a diagonal line defined by two coordinate data on a display screen. This rectangle is called hereinafter "clipping rectangle". When figure data of a first area are transferred, by use of the clipping function, to a second area which has one portion contained in a drawing-enable area defined by the clipping rectangle, only data of that portion of the second area should be replaced by the figure data of the first area. According to prior art, the coordinate data of every dot is compared with the above-mentioned two coordinate data to detect the portion of the second area overlapping the drawing-enable area defined by the clipping rectangle and the data of the portion of the second area are replaced one-bit by one-bit by the figure data of the first area. For this reason, a considerably long processing time is required.